There exists a large number of applications of hollow fiber membrane systems wherein large contact area between different phases moving relative to one another and low pressure drop are very important. Some examples are gas-liquid contactors for flue gas treatment and liquid-liquid contactors for waste water treatment as described in U.S. Pat. No. 5,169,529 to Carroll et al, U.S. Pat. Nos. 4,750,918 and 4,789,468 to Sirkar, U.S. Pat. No. 5,104,535 to Cote et al, and U.S. Pat. Nos. 4,959,152 and 5,174,900 to Nichols et al. Other examples are artificial blood oxygenators as described in U.S. Pat. No. 4,639,353 to Takemura et al, U.S. Pat. No. 4,659,549 to Hamada et al, U.S. Pat. No. 4,975,247 to Badolato et al, U.S. Pat. No. 5,037,610 to Fukasawa et al, and U.S. Pat. No. 5,124,127 to Jones et al, and artificial kidneys as described in U.S. Pat. No. 5,160,615 to Takagi et al.
One approach for meeting the low pressure drop requirement is by providing fibers of larger internal diameter but without increasing the outside diameter. But, this approach is detrimental to mechanical strength of the fibers. Another approach for meeting the large contact area is by providing more dense packing of fibers within a given volume. But, this approach is detrimental to uniformity of the flows outside of the fibers.
In natural membrane capillary systems, such as for example in human lungs, the problem of pressure drops of flowing fluids is solved by using extremely short capillaries. For example, the capillaries which supply blood to be contacted with lung sacs have internal diameter as low as seven micrometers but they exhibit low resistance to flow because they are extremely short, being about 100 micrometers in length. This is one of the important reasons why the natural systems are so efficient in mass transfer. Natural membrane capillary systems are good models to emulate in many artificial applications in industrial and medical fields.
No substantial technical problems exist in production of polymer permeable hollow fibers with solid or porous walls which are comparable in size to the capillaries of the natural systems. However, the lung system of mammals is built as a complicated multiple-level tree-like system which progresses from small to larger connecting passages. Heretofore, such natural systems have been viewed as too complex to replicate in artificial designs. Thus, up to the present time the emulation of natural systems has not been viewed by conventional thinking as a practical approach.